Semiconductor device

ABSTRACT

A semiconductor device such as a DPAM memory device is disclosed. A, Substrate ( 12 ) of semiconductor material is provided with energy band modifying means in the form of a box region ( 38 ) and is covered by an insulating layer ( 14 ). A semi-conductor layer ( 16 ) has source ( 18 ) and drain ( 20 ) regions formed therein to define bodies ( 22 ) of respective field effect transistors. The box region ( 38 ) is more heavily doped than the adjacent body ( 22 ), but less highly doped than the corresponding source ( 18 ) and drain ( 20 ), and modifies the valence and/or conduction band of the body ( 22 ) to increase the amount of electrical charge which can be stored in the body ( 22 ).

[0001] The present invention relates to semiconductor devices, and relates particularly, but not exclusively, to semiconductor charge storage devices such as semiconductor memories, and to substrates for manufacturing such devices.

[0002] International patent application PCT/EP02/06495 discloses a DRAM (Dynamic Random Access Memory) device comprising a matrix of memory cells, each cell being formed by a field effect transistor. By applying suitable voltage pulses between the gate and drain and between the source and drain of each transistor, an electric charge can be generated and stored in the body of the transistor, the presence or absence of the charge representing a “1” or “0” state of a binary data bit. Memory devices using SOI (Silicon On Insulator) type field effect transistors are disclosed in more detail in “SCI technology: materials to VLSI”, second edition, Kluwer, Boston 1997.

[0003] The transistors used in this type of device are PD-SOI (partially depleted silicon on insulator) transistors, which are formed in a layer of silicon formed on an insulating layer, the source, body and drain of each transistor being formed in the same layer, throughout the whole thickness of the silicon layer. The silicon layer is then covered by a dielectric film on which the gate of each transistor is formed.

[0004] To enable a charge to be stored in the body of a transistor of this type, it is necessary for the body of the transistor to have a sufficiently thick layer of silicon at its central Dart to provide the silicon with a non-depleted region, known as the neutral region, the charge being stored in, or in the proximity of, this latter region. As a consequence, such transistors are known as partially depleted transistors.

[0005] FD-SOI (fully depleted silicon on insulator) transistors are known, in which the silicon layer in which the source and drain regions are formed is thinner and/or the doping is less concentrated than in the case of partially depleted SOI transistors, as a result of which no neutral zone is provided. This means that it is not possible to store a charge in the body of such transistors. However, FD-SOI transistors present a number of advantages compared with partially depleted transistors, for example excellent short channel behaviour and a very rapid switching frequency. These advantages result from the small thickness of the silicon layer.

[0006] A comparison of the construction of bulk and SOI transistors is shown in FIGS. 1 and 2. Referring to FIGS. 1a and 2 a, an SOI transistor is formed from a substrate 10 having a substrate layer 12 of silicon, an insulating layer 14 of silicon oxide or sapphire, and a silicon layer 16. In the case of a transistor using bulk technology, as shown in FIGS. 1b and 2 b, the substrate 10 comprises only the silicon substrate layer 12.

[0007] Referring to FIGS. 2a and 2 b, in which parts common to both types of device are denoted by like reference numerals, integrated circuits comprising field effect transistors (of which only one is shown in each of FIGS. 2a and 2 b) are formed on the substrates 10 by successive photolithographic operations in which layers are partially removed, doped or new layers are deposited. The field effect transistor shown in FIG. 2a has a source 18 and a drain 20 formed in layer 16, a body 22 being defined between the source 16 and drain 20, the source 18 and drain 20 extending through the full depth of layer 16. As will be familiar to persons skilled in the art, the source 18, drain 20 and body 22 are formed by doping of the silicon of layer 16. The body 22 is covered by a dielectric film 24 overlapping with source 18 and drain 20, and on which a gate 26 is provided. The layer 25 is removed around the source 18 and drain 20, and replaced by an insulating framework 28 of silicon oxide. When a suitable voltage is applied to the gate 26, an electrically conducting channel 30 connecting the source 18 and drain 20 forms at the surface of the body 22 at the interface with dielectric film 24.

[0008] Referring to FIG. 2b, the source 18 and drain 20 are formed in substrate layer 12 to define the body 22 between the source and the drain, and the dielectric film 24 covers the body 22 and overlaps the source 18 and drain 20.

[0009] Referring now to FIGS. 3a and 3 b, FIG. 3b shows the variation of potential with thickness z (measured from the interface with dielectric film 24) of region B in the bulk transistor of FIG. 3a. The graph shown in FIG. 3b shows curves B_(c), B_(v) and N_(f), representing the potential of the valence band, the conduction band and the Fermi level respectively at the interior of body 22. Similarly, FIGS. 4a and 4 b show corresponding diagrams for a PD-SOI transistor, and FIGS. 5a and 5 b show corresponding diagrams for a FD-SOI transistor.

[0010] It can be seen from FIGS. 3b to 5 b that the valence band B_(v) and conduction band B_(c) have a minimum value of potential at the interface of body 22 and dielectric film 24 in the case of each type or transistor. However, it can be seen from FIG. 3b that the potentials B_(v) and B_(c) vary in a first zone Z_(d), and tend towards a limiting value beyond which a second zone Z_(n), known as the neutral zone, starts. This second zone Z_(n), extends through the total thickness of the substrate layer 12. When a suitable voltage is applied to gate 26, an electrically conducting channel 30 forms at the surface of the body 22 at the interface with dielectric film 24.

[0011] Referring now to FIG. 4b, it can be seen that the PD-SOI transistor has two depletion zones Z_(d1) and Z_(d2), adjacent dielectric film 24 and insulting layer 14 respectively, between which is located a neutral zone Z_(n). It will be appreciated by persons skilled in the art that the shape and extent of depletion zone Z_(d2) depends upon the back gate potential relative to the front gate potential. It is possible to store an electric charge in the body 22, in particular in, or in the proximity of, neutral zone Z_(n) located between the two depletion zones Z_(d1) and Z_(d2). One possible application of such a transistor is as an individual memory cell, capable of representing two logic states depending upon the presence or absence of charge in or in the proximity of the neutral zone Z_(n), for forming a semiconductor memory device such as a DRAM.

[0012] Referring now to FIG. 5b, it can be seen that in the case of the FD.SOI transistor, the potential of the conduction band B_(c) and valence band B_(v) varies continually throughout the entire thickness of body 22. In other words, the body 22 has a depletion zone Z_(d) extending throughout its entire thickness, as a result of which no neutral zone exists. It is therefore not possible to store charge in the body 22, and the transistor of this type therefore cannot be used as a memory cell. However, transistors of this type have a number of advantages, in particular excellent short channel behaviour and a very rapid switching frequency, these properties being as a result of the small thickness of layer 16.

[0013] Preferred embodiments of the present invention seek to combine the advantageous features of partially and fully depleted SOI transistors.

[0014] According to an aspect of the present invention, there is provided a semiconductor device comprising:

[0015] a substrate of semiconductor material;

[0016] a first electrically insulating layer provided on said substrate;

[0017] a first semiconductor layer provided on said first insulating layer and adapted to have respective source and drain regions of at least one field effect transistor formed therein to define a respective body region between said source and drain regions; and

[0018] energy band modifying means for modifying the valence and/or conduction band in a said body region of at least one said field effect transistor to increase the amount of electrical charge which can be at least temporarily stored in said body region.

[0019] By providing energy band modifying means for modifying the valence and/or conduction band of the body region of at least one said field effect transistor to increase the amount of electrical charge which can be stored in said body region, this provides the surprising advantage that the first semiconductor layer can be made considerably thinner than in the case of the prior art, which means that the advantages due to the thin layers of FD-SOI transistors (e.g. faster device performance) can be combined with the charge storing capability of PD-SOI transistors. For example, the present invention can be used to construct a particularly compact semiconductor memory device in which individual bits of data are represented by the presence or absence of charge stored in the body of individual transistors, while also utilising transistors of high performance.

[0020] The energy band modifying means may be adapted to create a region in which an electrical charge can be at least temporarily stored in the body region or at least one said field effect transistor;

[0021] In a preferred embodiment, the energy band modifying means is adapted to increase the length, in a direction substantially perpendicular to said first insulating layer, of a region in which the energy of said valence and conduction band in the body region of at least one said field effect transistor is substantially constant.

[0022] The energy band modifying means may be adapted to apply a respective voltage change to a respective source, drain and at least one gate of at least one said field effect transistor.

[0023] By applying voltage shifts to the source, drain and at least one gate of at least one said field effect transistor, this enables the charge storing capability of the invention to be achieved without significantly altering the substrate potential. This significantly improves the versatility of integrated devices formed from the device, for example by providing the advantage that charge storing memory devices can be located on the same substrate as logic devices, which latter devices generally cannot function if significant voltage shifts are applied to the substrate, and the voltage shifts applied to the memory devices only. The advantage is also provided that the device can be manufactured with the minimum number of manufacturing steps.

[0024] In a preferred embodiment, the energy band modifying means comprises a doped portion of the body region of at least one said field effect transistor, wherein said doped portion is more heavily doped than the adjacent portion of the corresponding said body region.

[0025] The doped portion may be arranged adjacent said first insulating layer.

[0026] The energy band modifying means may comprise at least one second semiconductor layer arranged adjacent source and drain regions of at least one said field effect transistor in use and on a side of said first insulating layer remote from said first semiconductor layer.

[0027] At least one said second semiconductor layer may at least partially cover said substrate.

[0028] At least one said second semiconductor layer preferably forms part of said substrate.

[0029] The device may further comprise contact means for connecting the or each said second semiconductor layer to a respective source of electrical input signals.

[0030] The device may further comprise respective source and drain regions of at least one said field effect transistor formed in said first semiconductor layer to define a respective body region between said source and drain regions.

[0031] The device may further comprise at least one respective gate or at least one said field effect transistor arranged adjacent the corresponding said body region of said field effect transistor.

[0032] The device may further comprise a second insulating layer formed on said first semiconductor layer, and at least one respective gate region of at least one said field effect transistor arranged on said second insulating layer.

[0033] In a preferred embodiment, the device is a semiconductor memory device.

[0034] According to another aspect of the present invention, there is provided a method of Controlling a semiconductor device comprising a substrate of semiconductor material, a first electrically insulating layer provided on said substrate, and a first semiconductor layer provided on said first insulating layer and having respective source and drain regions of at least one field effect transistor formed therein to define a respective body region between said source and drain regions, the method comprising modifying the valence and/or conduction band in a respective body region of at least one said field effect transistor to increase the amount of electrical charge which can be at least temporarily stored in said body region.

[0035] The step of modifying said valence and/or conduction band may comprise applying a respective voltage change to a respective source, drain and at least one gate of said field effect transistor.

[0036] The step of modifying said valence and/or conduction band may comprise applying a respective voltage to at least one second semiconductor layer located on a side of said first insulating layer remote from said first semiconductor layer.

[0037] According to a further aspect of the present invention, there is provided a semiconductor wafer comprising:

[0038] a substrate of doped semiconductor material;

[0039] a first electrically insulating layer provided on said substrate;

[0040] a first semiconductor layer provided on said first insulating; and

[0041] at least one second layer of doped semiconductor material provided on a side of said first electrically insulating layer remote from said first semiconductor layer, wherein the or each said second semiconductor layer is more strongly doped than the respective region of said substrate adjacent thereto.

[0042] Preferred embodiments of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

[0043]FIG. 1a shows a schematic cross-sectional view of a known substrate for fabrication of integrated circuits using SOI technology;

[0044]FIG. 1b is a schematic cross-sectional view of a known substrate for fabrication of integrated circuits using bulk technology;

[0045]FIG. 2a is a schematic cross-sectional view of a known transistor formed from the substrate of FIG. 1a;

[0046]FIG. 2b is a cross-sectional view of a known transistor formed from the substrate of FIG. 1b;

[0047]FIG. 3a is a schematic cross-sectional view of the transistor of FIG. 2b showing a region in which variation in potential is considered;

[0048]FIG. 3b is a graph illustrating the variation in potential in region B of the transistor of FIG. 3a;

[0049]FIG. 4a is a schematic cross-sectional view of the transistor of FIG. 2a using PD-SOI technology;

[0050]FIG. 4b is a graph showing the variation of potential in region B of the transistor shown in FIG. 4a;

[0051]FIG. 5a is a schematic cross-sectional view of the transistor or FIG. 2a using FD-SOI technology;

[0052]FIG. 5b is a graph showing the variation of potential in the region B of the transistor of FIG. 5a;

[0053]FIG. 6 is a schematic cross-sectional view of a semiconductor device of a first embodiment of the present invention.;

[0054]FIG. 7a is a schematic cross-sectional view of a semiconductor device of a second embodiment of the present invention, having unmodified energy bands;

[0055]FIG. 7b is a schematic cross-sectional view of the device of FIG. 7a having modified energy bands; and

[0056]FIG. 8 is a schematic cross-sectional view of a semiconductor device of a third embodiment of the present invention.

[0057] Referring to FIG. 6, a semiconductor device embodying the present invention is shown in which parts common to the device of FIG. 5a are denoted by like reference numerals. A first semiconductor layer in the form of a silicon layer 16 in which an NMOS transistor is formed is covered by an insulating layer 32, perforated by windows which are filled with conductive material to form contact areas 34, 35 connected to the source 18 and drain 20 regions respectively. The contact areas 34, 35 are connected to conductive lines 36, 37 respectively.

[0058] The NMOS transistor of FIG. 6 has a substrate layer 12 of n-type silicon having a second semiconductor layer in the form of a box region 38 of p-type silicon which is highly doped compared with the body 22, but less highly doped than the corresponding source 18 and drain 20. The box region 38 is connected to a conductive line 40 by means of contact area 41 passing through insulating layer 14, framework 28 and insulating layer 32. An arrangement of transistors of this type can be made using 0.13 μm technology, having an insulating layer 14 of thickness 400 nm and silicon layer 16 of thickness 30 nm. It should be noted that a single contact 41 can be used for a plurality of transistors of this type, and would typically be used for tens or hundreds of transistors.

[0059] By applying a negative voltage to box region 38, typically in the region of −20V for a layer 14 of thickness 400 nm, it is possible by means of the potential difference between gate 26 and box region 38 to form a neutral zone in the body 22, similar to the neutral zone of the PD-SOI transistor of FIG. 4. In this way, it is possible to generate, store and eliminate an electric charge in this neutral zone, the voltage necessary to create the neutral zone being reduced if the thickness of the layer 14 is reduced. Accordingly, such a transistor can be used to store a charge representing binary data, for example in a semiconductor DRAM memory or embedded memory device. A process for generating or eliminating electric charge in such a transistor is described in International patent application PCT/GB02/06495. In the regions of the device not provided with a box region 38, it is possible to form conventional FD-SOI or PD-SOI transistors.

[0060] Referring to FIGS. 7a and 7 b, in which parts common to the embodiment of FIG. 6 are denoted by like reference numerals, as an alternative to providing box region 38 to distort the valence and/or conduction band in the body 22 to increase the amount of charge which can be stored in the body 22, it is possible to have a sufficiently large potential difference between the gate 26 and substrate layer 12 adjacent the body 22 to provide a charge storing neutral zone in the body 22. This is achieved, for example, by applying a relatively large voltage shift (i.e. in the region of +20V) to each of the gate 26, source 18 and drain 20 of the transistor.

[0061] In particular, as shown in FIGS. 7a and 7 b, FIG. 7a illustrates a fully depleted (FD) SOI transistor biased in such a way as to behave like a normal FD device. In FIG. 7a, applying 2V on the gate 26 and 2V on the drain 20, while the source 18 is kept at 0V turns on the transistor. FIG. 7b, on the other hand, illustrates the same FD SOI transistor behaving electrically like a partially depleted (PD) device. The addition of 20V on all device nodes distorts the bands in such a way as to create a neutral region 23 in the transistor body 22. 20V is used when layer 14 is typically 400 nm thick. The respective voltage added is reduced proportionally in the case of a thinner layer 14.

[0062] The arrangement of FIG. 7b enables the charge storing neutral zone to be formed without the necessity of significantly altering the voltage of the substrate layer 12, and requires fewer manufacturing steps than are necessary for the device shown in FIG. 6. This also provides the advantage that memory and logic devices can be formed on the same substrate, since although memory devices generally function correctly if a significant voltage shift is applied to the substrate, this is generally not the case for logic devices such as processors. Accordingly, the voltage shifts are applied to the memory devices but not to the logic devices.

[0063] Referring to FIG. 8, in which parts common to the embodiments of FIGS. 6 and 7 are denoted by like reference numerals, the box region 38 is replaced by a highly doped region at the lower part of body 22 adjacent the insulating layer 14, the source 18 and the drain 20. Such a layer causes a similar effect to operation of the box region 38 of FIG. 6. In particular, the energy bands of the body of the transistor are modified by doping more heavily a portion 25 of the body 22. This creates a doping gradient in the body. The body 22 has therefore an upper part 21 with low doping content and a lower part 25 with a high doping content. This can be achieved by proper ion implantation, by epitaxy or by diffusion. The higher doping level of the lower body part 25 distorts the bands in such a way as to create a region where the bands are flat, therefore creating a neutral region 23.

[0064] It can therefore be seen that the present invention allows transistors to be formed, the bodies of which can store an electric charge representing a binary data bit, in a silicon layer much narrower than in the prior art, with the advantage that the improved device performance of FD-SOI transistors is obtained.

[0065] It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. Furthermore, it will be appreciated by persons skilled in the art that the above principle can be applied to PD-SOI type circuits when the thickness of the neutral zone is insufficient for the intended purpose or those circuits, and that the principle can be applied to PMOS type transistors as well as NMOS type transistors, in which case the polarities of the voltages used are opposite from those set out in the above-described embodiment. 

1. A semiconductor device comprising: a substrate of semiconductor material; a first electrically insulating layer provided on said substrate; a first semiconductor layer provided on said first insulating layer and adapted to have respective source and drain regions of at least one field effect transistor formed therein to define a respective body region between said source and drain regions; and energy band modifying means for modifying the valence and/or conduction band in a said body region of at least one said field effect transistor to increase the amount of electrical charge which can be at least temporarily stored in said body region.
 2. A device according to claim 1, wherein said energy band modifying means is adapted to create a region in which an electrical charge can be at least temporarily stored in the body region of at least one said field effect transistor.
 3. A device according to claim 1 or 2, wherein said energy band modifying means is adapted to increase the length, in a direction substantially perpendicular to said first insulating layer, of a region in which the energy of said valence and conduction band in the body region of at least one said field effect transistor is substantially constant.
 4. A device according to any one of the preceding claims, wherein said energy band modifying means is adapted to apply a respective voltage change to a respective source, drain and at least one gate of at least one said field effect transistor.
 5. A device according to any one of the preceding claims, wherein said energy band modifying means comprises a doped portion of the body region of at least one said field effect transistor, wherein said doped portion is more heavily doped than the adjacent portion of the corresponding said body region.
 6. A device according to claim 5, wherein said doped portion is arranged adjacent said first insulating layer.
 7. A device according to any one of the preceding claims, wherein said energy band modifying means comprises at least one second semiconductor layer arranged adjacent source and drain regions of at least one said field effect transistor in use and on a side of said first insulating layer remote from said first semiconductor layer.
 8. A device according to claim 7, wherein at least one said second semiconductor layer at least partially covers said substrate.
 9. A device according to claim 7 or 8, wherein at least one said second semiconductor layer forms part of said substrate.
 10. A device according to any one of claims 7 to 9, further comprising contact means for connecting the or each said second semiconductor layer to a respective source of electrical input signals.
 11. A device according to any one of the preceding claims, further comprising respective source and drain regions of at least one said field effect transistor formed in said first semiconductor layer to define a respective body region between said source and drain regions.
 12. A device according to claim 11, further comprising at least one respective gate of at least one said field effect transistor arranged adjacent the corresponding said body region of said field effect transistor.
 13. A device according to claim 12, further comprising a second insulating layer formed on said first semiconductor layer, and at least one respective gate region of at least one said field effect transistor arranged on said second insulating layer.
 14. A device according to any one of the preceding claims, wherein the device is a semiconductor memory device.
 15. A method of controlling a semiconductor device comprising a substrate of semiconductor material, a first electrically insulating layer provided on said substrate, and a first semiconductor layer provided on said first insulating layer and having respective source and drain regions of at least one field effect transistor formed therein to define a respective body region between said source and drain regions, the method comprising modifying the valence and/or conduction band in a respective body region of at least one said field effect transistor to increase the amount of electrical charge which can be at least temporarily stored in said body region.
 16. A method according to claim 15, wherein the step of modifying said valence and/or conduction band comprises applying a respective voltage change to a respective source, drain and at least one gate of said field effect transistor.
 17. A method according to claim 15, wherein the step of modifying said valence and/or conduction band comprises applying a respective voltage to at least one second semiconductor layer located on a side of said first insulating layer remote from said first semiconductor layer.
 18. A semiconductor wafer comprising: a substrate of doped semiconductor material; a first electrically insulating layer provided on said substrate; a first semiconductor layer provided on said first insulating layer; and at least one second layer of doped semiconductor material provided on a side of said first electrically insulating layer remote from said first semiconductor layer, wherein the or each said second semiconductor layer is more strongly doped than the respective region of said substrate adjacent thereto. 